[Xamarin.Android.Build.Tasks] Library project C# changes & Rebuilds (dotnet#764)

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=58865

Consider a project with an `App.csproj` and a referenced
`Lib.csproj`. Not all changes to C# sources within `Lib.csproj`
should require rebuilding and redeploying the `.apk` when
Fast Deployment is enabled; only those that result in changes
to Java Callable Wrappers should require rebuilding and
redeploying the `.apk`.

Unfortunately, that wasn't the case: *any* change to `Lib.csproj`
would result rebuilding and redeploying the `.apk`, because
`AndroidManifest.xml` was constantly being modified, which would
cause the `_CompileToDalvikWithDx` target to be invoked, resulting
in a `.apk` rebuild + redeploy.

The item

	<meta-data android:name="android.support.VERSION" android:value="25.4.0" />

was constantly being duplicated in the manifest. As a result it
was always newer and was triggering a change of build targets.

So we need to double check we are NOT adding 100% duplicate
values to the manifest.

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/ManifestTest.cs

@@ -486,6 +486,8 @@ public void MergeLibraryManifest ()
             android:authorities='${applicationId}.FacebookInitProvider'
             android:name='.internal.FacebookInitProvider'
             android:exported='false' />
+        <meta-data android:name='android.support.VERSION' android:value='25.4.0' />
+        <meta-data android:name='android.support.VERSION' android:value='25.4.0' />
     </application>
 </manifest>
 ", encoding: System.Text.Encoding.UTF8);
@@ -522,6 +524,8 @@ public void MergeLibraryManifest ()
 						"${applicationId}.FacebookInitProvider was not replaced with com.xamarin.manifest.FacebookInitProvider");
 					Assert.IsTrue (manifest.Contains ("com.xamarin.test.internal.FacebookInitProvider"),
 						".internal.FacebookInitProvider was not replaced with com.xamarin.test.internal.FacebookInitProvider");
+					Assert.AreEqual (manifest.IndexOf ("meta-data", StringComparison.OrdinalIgnoreCase),
+					                 manifest.LastIndexOf ("meta-data", StringComparison.OrdinalIgnoreCase), "There should be only one meta-data element");
 				}
 			}
 		}

src/Xamarin.Android.Build.Tasks/Utilities/ManifestDocument.cs

@@ -401,6 +401,16 @@ void MergeLibraryManifest (string mergedManifest)
 			}
 		}
 
+		void RemoveDuplicateElements ()
+		{
+			var duplicates = doc.Descendants ()
+			                    .GroupBy (x => x.ToFullString ())
+					    .SelectMany (x => x.Skip (1));
+			foreach (var duplicate in duplicates)
+				duplicate.Remove ();
+			
+		}
+
 		IEnumerable<XNode> FixupNameElements(string packageName, IEnumerable<XNode> nodes)
 		{
 			foreach (var element in nodes.Select ( x => x as XElement).Where (x => x != null && ManifestAttributeFixups.ContainsKey (x.Name.LocalName))) {
@@ -838,6 +848,7 @@ public void Save (string filename)
 
 		public void Save (System.IO.TextWriter stream)
 		{
+			RemoveDuplicateElements ();
 			var ms = new MemoryStream ();
 			doc.Save (ms);
 			ms.Flush ();

src/Xamarin.Android.Build.Tasks/Utilities/XDocumentExtensions.cs

@@ -22,6 +22,11 @@ public static string[] GetPaths (this XDocument doc, params string[] paths)
 				e = e.Elements (p);
 			return e.Select (p => p.Value).ToArray ();
 		}
+
+		public static string ToFullString (this XElement element)
+		{
+			return element.ToString (SaveOptions.DisableFormatting);
+		}
 	}
 }